One of the cheapest methods is to introduce a chemical directly into the tender-tanks. From here it passes to the boiler. Even this method is only suitable when the resulting precipitate will not cause a scale to form on the boiler-shell. At its best it requires frequent washing-out of the boiler. Such a method cannot be considered good practice, even though it is cheap.
When the impurities in the water consist chiefly of the carbonates of lime or magnesia which are held in solution, the treatment is very simple and inexpensive. These carbonates are only held in solution as long as the water is charged with carbonic-acid gas, as it always is under these conditions. If common lime is thrown into the water it unites with the free carbonic-acid gas and absorbs it. Since these carbonates are insoluble in water which is free from carbonic-acid gas, they are precipitated and the clear water may then be drawn off. The lime required for a water containing 40 grains of carbonates per gallon will cost less than one cent per thousand gallons. The cost of the labor may be more than this. A very few cents per thousand gallons will usually suffice for the cost of such purification. When the sulphate of magnesia is present the purifying is far more expensive, since it requires the use of sodium carbonate or "soda-ash" as a reagent. This is worth about one cent per pound, and the cost of the required amount for 1000 gallons will be about one cent for each eight grains of sulphate per gallon. Sometimes the water is so strongly impregnated that a very large amount of reagent is necessary, and the resultant water may become objectionable on account of "foaming".
A complete study of the precise chemical character of the water, together with the amount and cost of the necessary reagents, is the only wise method to pursue even before any source of supply is selected. Railroads have frequently found it wise to abandon sources of supply on which considerable money has already been spent, because it has been discovered that the water was selected without a proper appreciation of its disadvantages.
The maintenance of water in the tanks is accomplished chiefly in one of four ways: (1) gravity, which is impossible or impracticable, except under peculiarly favorable conditions; (2) by windmills, which are too uncertain, unless an extra-large storage capacity is provided, which reduces the economy of the method; (3) by steam-engines; and (4) by gasoline-engines. The first two methods need hardly be discussed. Under especially favorable circumstances they have their advantages of economy of maintenance if not of first cost. The methods of direct engine-pumping are the only standard methods which are everywhere applicable. The development of gasoline-engines during the last few years has resulted in a very large increase in the use of engines of this type. The great advantages of the gasoline-engine type are due to the low cost of the gasoline, which is usually not more than 10 c. per gallon, and, secondly, the facility with which the engines are run, even by a very low grade of unskilled labor. The economy is even greater when the size of the plant is very small, since very small steam-engines will use as much as 15 or 20 pounds of coal per horse-power hour. Under similar conditions a gasoline-engine will use about one-tenth of a gallon. Some comparative estimates made by the Chicago & Alton Railroad at several places found that the cost of pumping (using coal) was anywhere from 1.4 to 3.4 times as great as the cost when using gasoline. Of course the absolute cost per thousand gallons depends very largely on the height to which the water must be raised, as well as on the size of the plant, but in round numbers it may be said that the cost of pumping water per thousand gallons by steam will vary from 4 to 10 c. per thousand gallons, while the cost using gasoline will vary from 1« to 3 c.